Field of the Invention
The present invention relates to a negative active material for rechargeable lithium secondary batteries capable of inhibiting battery side reactions and gas generation and improving battery performance since moisture formed during an oxidation-reduction (redox) reaction is effectively absorbed into a surface of the negative active material, a method of preparing the same, and a rechargeable lithium secondary battery including the same.
Description of the Related Art
Rechargeable lithium secondary batteries (e.g., lithium ion batteries), nickel-hydrogen batteries, and other secondary batteries have been recognized to be of growing importance as vehicle-mounted power sources, or power sources for portable terminals such as laptop computers. In particular, rechargeable lithium secondary batteries which are lightweight and may have a high energy density may be desirably used as high-output power sources for vehicle mounting, and thus demand for rechargeable lithium secondary batteries is expected to increase in the future.
A rechargeable lithium secondary battery is manufactured by installing a porous separation film between a positive electrode and a negative electrode, followed by injecting a liquid electrolyte between the positive electrode and the negative electrode. Here, a material in which lithium ions are intercalatable and deintercalatable is used as the negative electrode or a negative active material and negative electrodes. In this case, electricity may be produced or consumed by a redox reaction caused by intercalation/deintercalation of lithium ions into/from the negative and positive electrodes.
Specifically, various types of carbon-based materials, in which lithium ions are intercalatable and deintercalatable and which include synthetic graphite, natural graphite, and hard carbon, have been applied as the negative active materials in the case of the rechargeable lithium secondary batteries. Among the carbon-based materials, graphite has a discharge voltage of −0.2 V lower than lithium, and thus secondary batteries in which graphite is used as a negative active material may have a high discharge voltage of 3.6 V. In addition, graphite has been most widely used since it may be advantageous in terms of the energy density of rechargeable lithium secondary batteries, and may ensure long lifespan of the rechargeable lithium secondary batteries due to excellent reversibility thereof. However, such graphite active materials have a problem in that they have low capacity with respect to the energy density of electrode plates per unit volume since graphite has a low density (a theoretical density of 2.2 g/cc), and side reactions with an organic electrolyte solution used at a high discharge voltage may easily occur upon manufacture of the electrode plates, resulting in swelling of the batteries, and thus battery capacity degradation.
To solve the above problems regarding such carbon-based negative active materials, Si-based negative active materials having a much higher capacity than graphite, and negative active materials using oxides such as tin oxide, lithium vanadium-based oxide, and lithium titanium-based oxide have been developed and researched.
However, the high-capacity, Si-based negative active materials undergo serious changes in volume during charge/discharge cycles, and thus lifespan characteristics may be deteriorated due to particle splitting.
In addition, oxide negative electrodes do not show satisfactory battery performance, and thus research on the oxide negative electrodes continues to be conducted. Among these, lithium titanium oxide (hereinafter referred to as “LTO”) does not form a solid electrolyte interface (SEI) layer due to poor reactivity with an electrolyte solution. Therefore, LTO is advantageous in terms of an irreversible reaction, and thus has very stable lifespan characteristics. Owing to excellent reversibility, LTO may also be desirably used to charge and discharge the secondary batteries at a high speed during intercalation/deintercalation of lithium (Li) ions. However, LTO has a high content of moisture, and thus has a drawback in that battery performance may be degraded due to the presence of moisture, and thus gas generation.
Therefore, there is a demand for development of methods capable of inhibiting generation of gases caused by moisture in the LTO-based negative active material itself, thereby preventing degradation of battery performance.